1. Field of the Invention
The present invention relates to motor drives, and more particularly to a motor drive with improved rotor angle estimation for greater efficiency.
2. Description of the Related Art
Most low cost motor drive systems employ encoderless (sensorless) control techniques. The rotor angle is estimated based on a motor model or motor back emf sensing, for example. The motor efficiency and torque per ampere capability can be significantly reduced if an incorrect rotor angle is used for controlling a permanent magnet synchronous motor (PMSM). Rotor angle estimation errors may be due to inaccuracies in estimator (angle) parameters or in motor back emf feedback sensing.
FIG. 2 is a graph illustrating motor output power versus rotor angle error for a surface-mounted permanent magnet motor operating at rated current and speed. As can be seen from FIG. 2, the output power is maximized when rotor angle error is zero.
A known optimization scheme of background interest is described in F. Abrahamsen et al., IEEE/IAS Transactions on Industry Applications, Vol. 37, No. 6 (November/December 2001), “Efficiency-Optimized Control of Medium-Size Induction Motor Drives,” incorporated by reference. In a related paper, “Energy Optimal Control of Induction Motor Drives,” Inst. Of Energy Technology, Aalborg University (February 2000), incorporated by reference, F. Abrahamsen proposes several measures for driving induction motors at variable speed, including motor drives with pulse-width modulated voltage source inverters.
Inverters for three-phase motor drives are well known in the industry. Typically a DC bus supplies switched power to different phases of an AC motor. With this type of motor control, it is desirable to accurately estimate rotor angle, for example on the basis of current feedback, for high performance control of the motor.
A rotor angle estimation scheme of background interest is described in the present inventor's Ser. No. 10/294,201 filed Nov. 12, 2002; and Ser. No. 60/465,890 filed Apr. 25, 2003, both incorporated by reference.